Conventionally, quantum dots have been manufactured using a dry chemical process, specifically, by inducing lattice mismatch between a substrate prepared in a vacuum and a layer deposited using a metal organic chemical vapor deposition (MOCVD) process. In this case, although nanoparticles may be formed and arranged on the substrate, expensive equipments are required. And it is also difficult to produce large quantities of quantum dots with a uniform size through conventional semiconductor fabrication methods. In order to solve these problems, a wet chemical process for synthesizing quantum dots with a uniform size using a surface active agent (surfactant) has been developed.
A method of manufacturing quantum dots through a wet chemical process includes preventing nanoparticles from aggregating together using a surfactant and controlling the reactivity of crystal surfaces by moderate choices of surfactants to synthesize quantum dots with various shapes with an uniform size. In 1993, the Bawendi group succeeded in synthesizing cadmium selenide (CdSe) quantum dots with a uniform size by means of a wet chemical process using trioctylphosphineoxide (TOPO) and trioctylphosphine (TOP) as surfactants, dimethyicadmium (Me)2Cd and selenium as precursors of Group II and Group VI. Also, the Alivisatosgroup developed a method of synthesizing CdSe quantum dots in a safer manner using hexadecylamine (HDA), trioctylphosphineoxide, and trioctylphosphine as surfactants, cadmium oxide (CdO), and TOPSe as precursors of Group II and Group VI.
Subsequently, a substantial amount of research has been conducted to passivate the surfaces of CdSe quantum dots with compound semiconductor shaving a larger bandgap in order to improve luminous characteristics and optical and environmental stability of the CdSe quantum dots. For example, CdSe/ZnS (refer to J.Phys.Chem.B, 1997, 101, 9465-9475), CdSe/ZnSe (refer to Nano Letters, 2002, 2, 781-874), CdSe/CdS (refer to J. Am. Chem, Soc., 1997, 119, 7019-7029), and ZnSe/ZnS (Korean Patent Registration No. 10-0376403) have been proposed.
However, in conventional core-shell quantum dots, when a thick shell is formed, an interface between the core and shell of the quantum dot becomes unstable due to the lattice mismatch between a core semiconductor material and a shell semiconductor material, thereby causes formation of defects which lower quantum efficiency of quantum dots. Thus, the conventional core-shell quantum dots have thin shell structures. As a result, the shell semiconductor material functions only to stabilize the surface state of the core of the quantum dot and cannot funnel electrons and holes to the core. Therefore the conventional core-shell quantum dots have limitations in luminous efficiency, optical stability, and environmental stability.
Therefore, research into the manufacture of a multishell including an intermediate shell has been progressed in order to minimize lattice mismatch between core structure and shell structure of quantum dots. For example, CdSe/CdS/ZnS (Korean Patent Registration No. 10-2005-0074779) and CdSe/CdS/Zn0.5Cd0.5S/ZnS (refer to J. Am. Chem. Soc. 2005,127,7480˜7488) were proposed. Since the core-multishell quantum dots described above have high luminous efficiencies and sufficient optical and environmental stability, they can be applied to practical applications such as various emission materials, laser materials, and biological marker materials. However, the manufacture of conventional core-multishell quantum dots involves complicated synthetic processes. Specifically, after each synthetic steps (i.e., synthesis of cores or each intermediate shells of quantum-dot), the synthetic process should be ended, all surfactants and precursors should be cleaned away, and a precursor required for intermediate shells should be injected into a new surfactant at an appropriate temperature to react over a long period. Also, during the sequential purification steps, some quantum dots and nonreacted precursors are removed too, which is counterproductive. Furthermore, in quantum dots having a stacked shell structure obtained according to the synthesis process described above, only excitons generated in quantum-dot cores are used, while excitons generated in quantum-dot shells do not contribute to light emission, which limits luminous efficiency of quantum dots.
Accordingly, there is still room to develop highly crystalline quantum dots having high luminous efficiency and optical and environmental stability, and a straightforward technique for synthesizing the quantum dots in large quantities at low cost.